This chapter talks about the types of transmission lines and network
connections, the electromagnetic spectrum, and what bandwidth is all about in
this emerging broadband era. It looks at the differences between analog and
digital signals, and it discusses multiplexing. Finally, this chapter describes
the various standards bodies and their roles in shaping aspects of
telecommunications.

This chapter is from the book

Transmission Lines

Two prerequisites must be satisfied to have successful communication. The
first prerequisite is understandability. The transmitter and receiver must speak
the same language. It doesn't matter how big or how clean a pipe you have
between the two endpoints. If they're not speaking the same language, you
will not be able to understand the message. In the case of data communications,
we've resolved these issues quite elegantly: We have software and hardware
translation devices that can convert between the different languages that
individual computing systems speak. In the realm of human communications,
we're about to embark on that exciting journey as well. Through the use of
advanced voice-processing systems, in the next five to seven years we should
have the ability to do real-time foreign language translation as part of the
network service.

The second prerequisite is the capability to detect errors as they occur and
to have some procedure for resolving those errors. In the case of human
communications, intelligent terminals at either endhuman beingscan
detect noise that may have affected a transmission and request a retransmission,
thereby correcting for that error. In the case of data devices, similar logic
has to be built in to end devices so that they can detect errors and request a
retransmission in order to correct for the errors.

If these two prerequisitesunderstandability and error controlare
met, then communication can occur. We communicate by using data devices over
what is generically termed a transmission line. There are five main types
of transmission linescircuits, channels, lines, trunks, and virtual
circuitseach of which has a specific meaning. The following sections
describe each of these types of transmission lines in detail.

Circuits

A circuit is the physical path that runs between two or more points.
It terminates on a port (that is, a point of electrical or optical
interface), and that port can be in a host computer (that is, a switching device
used to establish connections), on a multiplexer, or in another device, as
discussed later in this chapter.

In and of itself, a circuit does not define the number of simultaneous
conversations that can be carried; that is a function of the type of circuit it
is. For example, a simple, traditional telephone circuit is designed to carry
just one conversation over one physical pathway. However, converting that to a
digital circuit gives you the ability to extract or derive multiple channels
over that circuit, subsequently facilitating multiple simultaneous
conversations. So, the circuit is the measure of the physical entity.

There are two types of circuits: two-wire circuits and four-wire
circuits.

Two-Wire Circuits

A two-wire circuit has two insulated electrical conductors. One wire is used
for transmission of the information. The other wire acts as the return path
to complete the electrical circuit. Two-wire circuits are generally deployed
in the analog local loop, which is the last mile between the subscriber and
the subscriber's first point of access into the network. Figure
2.1 shows an example of a two-wire circuit.

Two-Wire and Four-Wire Versus Two-Pair and Four-Pair

Don't confuse the terms two-wire circuit and four-wire circuit with the
terms two-pair and four-pair. Two-pair and four-pair refer to the number of
wires in the internal cabling plan. Two-wire and four-wire have to do with the
number of electrical conductors associated with a transmission line.

Four-Wire Circuits

A four-wire circuit has two pairs of conductors. That is, it has two sets of
one-way transmission paths: one path for each direction and a complementary
path to complete the electrical circuit (see Figure
2.2). Four-wire circuits are used where there is distance between the termination
points which requires that the signal be strengthened periodically. So, for
example, four-wire circuits connect the various switches that make up the public
switched telephone network (PSTN). Four-wire circuits are also used with leased
lines, where a customer may be connecting locations of its own that are separated
by distance. Also, all digital circuits are provisioned on a four-wire basis.

There are two types of four-wire circuits: physical four-wire and logical
four-wire. In physical four-wire you can actually count four wires. In
logical four-wire, physically there are only two wires, but you derive
the four individual paths by splitting the frequency. Half of the frequency band
carries the transmit signal, and the other half carries the receive signal. So
you can't always tell just by looking what kind of circuit you're
dealing with; the application dictates the type of circuit it is.

Using Two-Wire and Four-Wire Circuits

Whenever you release energy into space, it loses power as it's traveling
over a distance. So, because networks were designed to carry communications over
a distance, we need tools to augment signals that have been losing power as they
have traveled across the network, which are called attenuated signals.
These tools are called amplifiers and repeaters. An amplifier boosts an
attenuated signal back up to its original power level so it can continue to make
its way across the network. The PSTN traditionally used copper wires. Based on
how quickly the signals flow through the copper wires, there's a certain
distance requirement between amplifiers. The distance requirement between
amplifiers is relatively short on copper wiresgenerally about 6,000 feet
(1,800 meters). As networks were built, these distance considerations were kept
in mind. (Repeaters are discussed later in this chapter, in the section
"Digital Transmission.")

Network builders had to give some thought to another aspect of amplifiers:
First-generation amplifiers were unidirectional. They could only amplify a signal
moving in one direction, so any time you needed to provision a circuit that
was going to be crossing a distance, you had to literally provision two circuitsone
to amplify the information in the transmit direction and a second to amplify
the information in the receive direction. Therefore, whenever a network was
crossing a distance, it needed to use a four-wire circuit. But in building out
the millions of local loops for subscribers, it was seen as being cost-effective
to have to pull only two wires into every home rather than four. Therefore,
the local loops were intentionally engineered to be very short; some 70% to
80% of the local loops worldwide are less than 2 miles (3.2 kilometers) long.
Because the local loops are short, they don't need amplifiers, and therefore
the subscriber access service can be provisioned over a two-wire circuit. However,
the local loop is increasingly being digitalized, so as we migrate to an end-to-end
digital environment, everything becomes four-wire. Figure
2.3 shows an example of a segment of a network in which two- and four-wire
circuits are traditionally used.

Channels

A channel defines a logical coversation path. It is the frequency
band, time slot, or wavelength (also referred to as lambda) over which a
single conversation flows. A channel is a child of the digital age because
digital facilities enable multiple channels. The number of channels on a
transmission line determines the number of simultaneous conversations that can
be supported. Because we are becoming more digitalized all the time, you often
hear people refer to the number of channels rather than the number of
circuits.

Lines and Trunks

Lines and trunks are basically the same thing, but they're used in
different situations. A line is a connection that is configured to
support a normal calling load generated by one individual. A trunk is a
circuit that is configured to support the calling loads generated by a group of
users; it is the transmission facility that ties together switching systems. A
switching system is a device that connects two transmission lines
together. There are two major categories of switching systems:

CPE switchesThe most prevalent form of switch in the
customer premises equipment (CPE) environment is the private branch exchange
(PBX), which is called a private automatic branch exchange (PABX) in some parts
of the world. A PBX is used to establish connections between two points. It
establishes connections between telephones that are internal to the
organization, and it establishes connections between internal extensions and the
outside world (that is, the PSTN).

Network switchesA hierarchy of network switches has evolved
over time, and the appropriate switch is called into action, depending on
which two points the switches are connecting together. For example, in Figure
2.4 the CPE is on the left-hand side. Each individual single-line instrument
represents a subscriber line. (Again, the fact that it's called a line
means that it's a circuit configured to carry the calling load of just
one user.) Above the single-line instrument is a business enterprise with
a PBX. The connection from this PBX to the PSTN occurs over a trunk that
is specifically configured to carry the calling load of multiple users.
Beyond the PBX are multiple end users that are attached to that PBX. Each
end user's connection would be referred to as a station line,
again emphasizing that the line is carrying the calling load of one user.

The customer environment attaches to the PSTN, and the first point of access
is the local exchange, which is also referred to as a Class 5
office (and in North America, as a central office). The traditional
local exchange switch can handle one or more exchanges, with each exchange
capable of handling up to 10,000 subscriber lines, numbered 0000 to 9999. The
only kind of call that a local exchange can complete on its own, without
touching any of the other switches in the network, is to another number in that
same local exchange. Local exchanges are discussed in detail in Chapter 5,
"The PSTN."

For a local exchange to call a neighbor that resides 10 miles (16 kilometers)
away and who draws a dial tone from a different local exchange, the connection
between those two different exchanges is accomplished through the second part of
the hierarchya tandem switch (also called a junction
exchange). The tandem switch is used to connect local exchanges throughout
the metropolitan area. When it's time to make a toll call, one that is
long-distance in nature, another switching center is called into actionthe
toll center (also called the Class 4 office, transit switch,
or trunk exchange). The toll center is responsible for establishing
and completing national, long-distance communications.

The top of the hierarchy is the international gateway, whose
exchanges are specifically designed to connect calls between different
countries.

A trunk supplies the connections between the numerous switches within the
PSTN, between customer-owned switches such as the PBX, and between the PBXs and
the PSTN. On the other hand, a line supports a single user in the form of a
subscriber line in the PSTN or an extension provisioned from the PBX. (Chapter 5
describes in detail the entities involved in managing local, tandem, and toll
exchanges.)

Virtual Circuits

Today, because of the great interest in and increased use of packet switching,
most networks use virtual circuits. Unlike a physical circuit, which terminates
on specific physical ports, a virtualcircuit is a series of logical
connections between sending and receiving devices (see Figure
2.5). The virtual circuit is a connection between two devices that acts
as though it's a direct connection, but it may, in fact, be composed of
a variety of different routes. These connections are defined by table entries
inside the switch. A connection is established after both devices exchange agreement
on communications parameters that are important to establishing and maintaining
the connection and on providing the proper performance for the application they
are supporting. The types of communication parameters that could be included
are message size, the path to be taken, how to deal with acknowledgements in
the event of errors, flow-control procedures, and error-control procedures.
The term virtual circuit is largely used to describe connections between
two hosts in a packet-switching network, where the two hosts can communicate
as though they have a dedicated connection, although the packets may be taking
very different routes to arrive at their destination.

PVCs

A PVC is a virtual circuit that is permanently available; that is, the
connection always exists between the two locations or two devices in question. A
PVC is manually configured by a network management system, and it remains in
place until the user reconfigures the network. Its use is analogous to the use
of a dedicated private line because it provides an always-on condition between
two locations or two devices.

SVCs

In contrast to PVCs, SVCs are set up on demand. They are provisioned
dynamically by using signaling techniques. An SVC must be reestablished each
time data is to be sent, and after the data has been sent, the SVC disappears.
An SVC is therefore analogous to a dialup connection in the PSTN. The main
benefit of an SVC is that you can use it to access the network from anyplace.
The predominant application for SVCs is to accommodate people who are working at
home, in a hotel, at an airport, or otherwise outside the physical location of
the enterprise network.